The invention relates to a viscous shear coupling comprising a housing filled with a viscous fluid, an inner part such as a shaft extending into the housing, and intermeshing inner and outer disks alternately and non-rotatably connected to the shaft and to the housing, wherein the width of the gap between adjacent disks is adjustable by means of a piston.
Such adjustable viscous shear couplings are used in allwheel drive motor vehicles in order to adapt the torque transmitted to the second, driven axle to the prevailing travel conditions. The adaptation is accomplished by changing the distance between the disks and/or by changing the degree to which the housing is filled with viscous fluid.
Adjustable viscous shear couplings are different from nonadjustable ones mainly in that no steep rise of the transmitted torque occurs in the latter as a result of a rise in heat and pressure in the coupling (the so-called "hump"). In known adjustable viscous shear couplings, elastic spacing elements are provided between the individual disks of a disk pack in order to ensure uniform disk spacings even when the individual disks are shifted axially, and in order to avoid friction-producing contact between individual disks. In the viscous shear coupling known from AT-PS 384,086 (U.S. No. 4,844,219), the elastic spacing elements are Belleville spring washers.
Such spacer elements are of considerable height, thus reducing the extent of the overlap of the inner and outer disks for a given housing diameter. Because of this, the effective surfaces of the disks and the transmitted torque are reduced. The elastic spacer elements also have the great disadvantage that they oppose the adjusting piston with a force which increases as the gap width is reduced, so that a greater force is required to effect the same adjustment. Furthermore, a thrust bearing which takes up space and is subject to wear must be installed in the housing to transmit the piston movement to the inner disks. Finally, the area outside the spring washers must also be considered as being dead space with regard to the degree to which the coupling is filled. Therefore, a larger adjustment of the gap width is required to compensate for this dead space.
It is known from DE-PS 39 28 975 to provide spacer rings between the disks of a set in order to reduce this dead space and to facilitate assembly of the coupling. However, this patent relates to a non-adjustable viscous shear coupling. The disks of one of the two disk packs cannot be shifted axially. Therefore, the intermediate rings act only as spacers between these fixed disks. An axial movement of the disks of the other disk pack only occurs during the "hump" when the axially displaceable disks are pressed against one of the two adjoining fixed disks. The disks are open-worked in order to promote the reaching of the "hump." Seals between movable disks and spacer rings are not formed because they would interfere with the movable disks being pressed against the fixed disks in this manner.
The present invention has as its object to ensure the maintenance of equal distances between the disks without utilizing mechanical spring elements and thus to reduce the size of an adjustable viscous shear coupling and to make it possible to operate with as small an adjusting force and as short an adjusting path as possible.